The correlation between increased intraocular pressure (IOP) and loss of sight due to glaucoma diseases has been acknowledged for some time. As a result, determining intraocular pressure is integral to ophthalmic examinations.
Various methods and corresponding devices, or tonometers, for determining intraocular pressure are known. These include invasive methods, wherein pressure sensors are inserted directly into the eye, and non-invasive methods which may involve a measurement apparatus which contacts the eye directly during measurement, or contactless measurement apparatus such as air-puff type tonometers which measure corneal deformation in response to a stream of air blown onto the surface to the eye.
In applanation tonometry the intraocular pressure is inferred from the force required to applanate or flatten a constant area of the cornea. The Maklakoff tonometer was an early example of an applanation tonometer, while the Goldmann tonometer is the most widely used version in current practice. An advantage of Goldmann applanation tonometry is that the tonometer is mounted on a slit lamp microscope, providing a stable base from which to handle the instrument and take measurements. However, the slit lamp mount is also disadvantageous since it means that the Goldmann tonometer is inherently not portable. Another disadvantage is that fluorescein dye and a topical anaesthetic must be introduced onto the surface of the eye. The fluorescein dye aids in viewing the mires and the anaesthetic is required since the tip of the device touches the cornea.
Indentation tonometry measures the depth of corneal indentation caused by a small plunger carrying a known weight. The indentation of the corneal surface is indirectly proportional to the intraocular pressure. For very high levels of intraocular pressure, extra weights can be added to cause the plunger to apply additional pressure to the cornea. The extent of movement of the plunger is measured using a calibrated scale.
The Schiøtz tonometer is the most common device and is based on the indentation tonometry principle. The Schiøtz tonometer consists of a curved footplate, which is placed on the cornea of a supine patient. A weighted plunger attached to the footplate sinks into the cornea. A scale provides a reading depending on how much the plunger sinks into the cornea, and a conversion table converts the scale reading into intraocular pressure measured in mmHg.
A limitation of the Schiøtz tonometer is that it must be used on a supine patient, so that the device is in the vertical position during use. Moreover, the weights must be adjusted for different intraocular pressure ranges. The Schiøtz tonometer can be difficult to use, particularly for operators with limited training.
Objects and advantages of the invention will become apparent to those of ordinary skill in the art having reference to the following specification together with its drawings.